6 – 1 INTRODUCTION
Illumination is defined as the intensity of light per unit area. When we talk of illumination, or simply lighting, we are referring to man made lighting. Daylight being excellent is not included, thus, we assume a night time condition.
Electric Illumination is the production of light by means of electricity and its applications to provide efficient, comfortable and safe vision. Specifically, when one speaks of lighting design, he refers to only two things:
1. The quantity of light
2. The quality of light
Quantity of Light – refers to the amount of illumination or luminous flux per unit area.
Quantity of light can be measured and easily handled because it deals with the number of light fixtures required for a certain area.
Quality of Light – refers to the distribution of brightness in the lighting installation. It deals with the essential nature or characteristics of light. In short, quality of light is the mixture of all the items related to illumination other than the quantity of light which includes several elements such as:
2. Brightness ratio or contrast
7. Psychological reaction to color and fixtures
There are four factors that affect illumination, namely:
Brightness is the light that seems to radiate from an object being viewed. Brightness or luminance is the luminous flux (light) emitted, transmitted or reflected from a surface.
Contrast is the difference in brightness or the brightness ratio between an object and its background. The recommended brightness ratio between an object being viewed and its background is normally 3:1.
If a print on a white paper can be clearly seen on a light background, it is due to the effect called contrast. Likewise, if a light object is placed on a dark background, the light object reflects more light and looks brighter although bought have equal illumination. It is for this reason that office furniture are generally light colored, tan or light green for eye comfort.
Glare is a strong, steady, dazzling light or reflection. There are two types of glare:
1. Direct Glare is an annoying brightness of light in a person’s normal field of vision.
2. Indirect or Reflected Glare is much more serious and difficult to control. Technically, reflected glare is a glossy object.
Diffuseness refers to the control of shadows cast by light. Diffuseness is the degree to which light is shadowless and is therefore a function of the number of directions to which light collides with a particular point and the comparative intensities.
Perfect Diffusion is an equal intensities of light clashing from all directions producing no shadows. A single lamp will cast sharp and deep shadows. A luminous ceiling provides a satisfactory diffuse illumination and less shadows.
The color of lighting and the corresponding color of the object within a space is an important consideration in producing a quality of light.
There are three characteristics that define a particular coloration. They are:
b. Brilliance or Value – is the difference between the resultant colors of the same hue, such as: white is the most brilliant of the neutral colors while black is the last.
c. Saturation or Chroma – is the difference from the purity of the colors. Colors of high saturation must be used in well lit spaces.
6 – 2 ESTIMATING ILLUMINATION AND BRIGHTNESS
In many respect, it is more important to know luminance measurements than illumination because the eye is more sensitive to brightness than simple illumination. However, it is more difficult to measure luminance than illumination.
There are three types of luminance meter, namely:
1. The Comparator type which requires the operator to make a brightness equivalence judgment between the target and the background.
2. The Direct
3. The Accurate Laboratory Instrument which unsuitable for field work.
The quantity of light level of illumination can be easily measured or calculated with the aid of portable foot candle meter.
Footcandle (fc) is the amount of light flux density. It is the unit of measure used when describing the amount of light in a room and is expressed in lumens per square foot.
Footlambert (fl) is defined as “the luminance of a surface reflecting. Transmitting or emitting one lumen (lm) of illumination per square foot of area in the direction being viewed or the conventional unit of brightness or luminance. In the same manner, the lumens (lm) is the light output generated continuously by a standard wax candle
In our study of light, we are interested in the amount of light that fall on the areas that we want to illuminate. We also want to know the lumens per square foot or square meter in a space.
This quantity called Light Flux Density is the common term Foot-candle (fc) represented by the formula:
Footcandle = Lumens
ILLUSTRATION 6 – 1
A 40 – watt fluorescent lamp 120 centimeters long produces 3,200 lumens of light in a room having a general dimension of 10 x 20ft. Find the illumination on the floor.
Footcandle = Lumens
fc = 3,200 lm. = 16 footcandle
10 x 20 ft.
The footcandle is an important unit of measure in calculating the desired illumination and layout of fixtures. In the absence of Tables of equivalent footcandles for a particular fixture, a rule of thumb of 10-30-50 illumination level is here presented.
10 – footcandle is adequate for halls and corridors
30 – footcande is sufficient for areas between work stations such as in offices other than desk areas.
50 – footcandle is satisfactory on spaces where office work is done.
However, providing an adequate quantity of light alone is not a guarantee for an efficient and comfortable vision. In fact, the quality of light is very important especially where difficult visual needs are required. The luminance or brightness of a diffusely reflecting surface is equal to the product of the illumination and the reflectance. Thus;
Luminance = Illumination x Reflectance factor or
Footlambert = Footcandle x Reflectance factor
ILLUSTRATION 6 – 2
From illustration 6 – 1, find the luminance if the reflectance factor of the wall is 40%.
1. Footlambert = Footcandle x Reflectance factor
= 16 x 40% = 6.4
Metric Lighting Units
In English System of measure, the distance is expressed in feet and the area is in square feet. Under the Metric System (SI) the distance and area are expressed in meters and square meters respectively.
Meanwhile, Lumens flux remains in Lumens but illumination or light flux is expressed in Lux. Thus:
Lux = Lumens
Table 6 -1 APPROXIMATE REFLECTANCE FACTOR
In the metric system, Luminance or Brightness is expressed in Lambert which is defined as “the luminance or brightness of a surface reflecting, transmitting or emitting one lumen per square centimeter. Millilambert is more conveniently used than the lambert because the value of lambert is greater than what is usually encountered.
Table 6 – 2 TABLE OF COMPARISON
ILLUSTRATION 6 – 3
A 40 – watts x 120 centimeters long fluorescent lamp produces 3,200 lumens of light in a room having a general dimension of 10ft. x 20ft. Compute the illumination on the floor comparing the English and the Metric units.
SOLUTION BY COMPARISON
English Metric (SI)
Light Flux = 3,200 lm. …………………. 3,200 lm
Area = 10’ x 20’ …………………. 10 x 20
= 200 sq. ft. ………………… 18.59 sq. m.
Illumination = 16 fc ……………………… 172.16 lux
Convert: 10 feet to meter = 3.048 m.
20……………. = 6.097 m.
Lux = 3,200 = 172.19 Lux
3.048 x 6.097
ILLUSTRATION 6 – 4
Compute for the brightness of a fixture with a 1’x 4’ plastic diffuser having a transmittance of .6 and illuminated by 2 pieces 3,200 lm. lamp assuming 100% use of light flux.
1. Luminance = Total lumens x transmission factor
Area of diffuser
= 2pcs. x 3,200 x .6
1’ x 4’
= 960 footlambert
1. To obtain the metric equivalent, multiply:
Millilambert = Footlambert x 1.076
= 960 x 1.076
= 1032.96 millilambert
The Watts per Square Meter
Another method used in determining the illumination is the watts per square meter wherein the floor area is computed from the outside dimensions of the building excluding open porches.
Depending upon the size of the room, color of wall and ceiling, types of lighting units and methods of lighting used, the watts per square meter method is may produce 50 to 100 lux which is approximately 5 to 10 footcandles.
1. Twenty watts (20) per square meter will provide an illumination of 100 to 150 lux which is approximately 10 to 15 fc in industrial areas.
2. For commercial areas, two (2) watts per square foot or 22 watts per square meter is will provide from 80 to 120 lux when used with standard quality equipment.
3. Forty (40) watts per square meter will provide about 200 lux which is approximately 20 fc wherein greater illumination is required
4. Sixty (60) watts per square meter will provide about 300 lux or approximately 30 fc which is recommended for many conventional, industrial and commercial requirements.
5. Eighty (80) watts per square meter will provide from 300 to 350 lux and in excess of supplementary lighting is necessary.
6 – 3 COEFFICIENT OF UTILIZATION AND MAINTENANCE FACTOR
The usable Initial footcandle or lux is equal to the footcandle produced by the coefficient of utilization (cu).
Initial was emphasized because the output is of a light fixture is reduced with time as the lamp fixture is becomes old and dirty. Lamp output normally drops and is termed as Maintenance factor (mf). And to find the average maintained illumination, we reduce the initial illumination by the maintenance factor.
The efficiency of a light fixture is equals the ratio of fixture output lumens to lamp output lumens. What we need is to determine a number indicating the efficiency of the fixture room combination, or how a particular light fixture lights a particular room. This number is normally expressed in decimal value called coefficient of utilization represented by letter (cu).
The usable initial footcandle is equal to the footcandle produced by the coefficient of utilization (cu).
a.) Initial Footcandle = footcandle x cu.
b.) Maintenance illumination = Lamp lumens x cu x mf
* Lamp lumen therefore is simply the rated output of the lamp.
TABLE 6 – 3 COEFFICIENT OF UTILIZATION
TABLE 6 – 4 MAINTENANCE FACTOR
ILLUSTRATION 6 – 5
A school classroom with a general dimension of 24 x 30 ft. is lighted with 10 fluorescent of 4F 40 T12 WW rapid start lamp. Calculate the initial and maintained illumination in footcandles (English) and Lux (Metric) assuming that (cu) is 0.35 and (mf) is 0.70.
SOLUTION – 1 (English Measure)
1. Refer to Table 5 – 3. An F 40 T12 WW watts fluorescent lamp has 3,200 lm. output. Multiply:
Lamp lumens = 10 fixturesX 4 lamps per fixture X 3,200 lumens per lamp
= 128,000 lumens
Initial footcandle = 128,000 x 0.35
24 x 30 ft.
= 62.22 fc x 0.70 mf
= 43.55 footcandle
SOLUTION – 2 By Metric Measure (SI)
Convert feet to meter: 24 ft. = 7.32 m.
30 ft = 9.14 m.
Lux = Lumens x cu x mf
= 10 x 4 x 3,200 x 0.35 x 0.70
7.32 m. x 9.14 m.
= 468.75 lux
Check the answer:
One lux = .09294
468.75 x .09294 = 43.56 fc.
Sometimes when the size of the room and the footcandle are given, the problem is how to find the number of lamps required in each fixture. The following example is offered.
ILLUSTRATION 6 – 6
An office room having a general dimension of 8 x 20 meters is to be lghted at an averaged maintained footcandle of 50 fc, How many 3-lamp fixtures of 120 centimeters long F40 T12 WW rapid start fluorescent lamps are required assuming the cu is 0.38 and the mf is 0.75?
1. Lamp lumens = maintained footcandle x area
cu x mf
= 50 fc x (8m. x 20 m.)
0.38 x 0.75
= 28,070 lumens
2. Each 40 watt fluorescent lamp has an output of 3,200 lumens, the number of lamps will be:
Number of lamps = 28,070
= 8.77 lamps
3. Since there are 3 lamps for each fixture, divide:
8.77 = 2.93 say 3 lamps in ach fixture
Calculation involving a wide area is more confusing than by computing the number of lamp fixtures per bay or per row which is more meaningful and interesting.
This could be done easily by using the following formula:
Number of mixtures = Illumination x area
Lamp per fixture x lumens x cu x mf
This means that the area lighted by a single area is:
Area per fixture = lamp per fixture x lumens per lamp x cu x mf
TABLE 6 – 6 EFFICACY OF VARIOUS LAMPS
ILLUSTRATION 6 – 7
An entire office floor is lighted at an averaged maintained 538 lux or 50 fc. The floor measures 20 meters by 50 meters and is divided into bays measuring 4 m. x 5 m. Using 2-lamp of F40 T12 CW rapid start preheat lamp, find the number of fixtures required. Assume an economy grade fixture with a lo cu of 0.35 and mf of 0.70
SOLUTION – 1
Solve for the number of fixtures per bay.
Refer to Table 5-3 for F40 T12 CW = 3,150 lm.
No. of Fixtures = Illumination x area
Lamp per fixture x lumens x cu x mf
Fixtures = 538 lux x (4 m. x 5m. )
2-lamps x 3,150 lm. x 0.35 x 0.70
= 10,760 = 6.9 fixtures
Accept 6 pieces per bay to make it symmetrical
SOLUTION – 2
1. From the following Formula, substitute the values:
Area per Fixtures = Lamp per fixture x lumen/ lamp x cu x mf
Fixtures = 2-lamps per fixture x 3,150 lm. x 0.35 x 0.70
= 1,543.5 = 2.87 sq.m. per fixture
2. Therefore, the number of fixture per bay is:
4 m x 5m = 6.9 say 6 pcs. Per bay for symmetry
6 – 4 MEASURING FOOTCANDLE
The unit measure of illumination is the footcandle or lux in the Metric System which is frequently used when describing the amount of light in a room. It is not enough to know how to calculate the illumination level but it is also equally important to know how to measure them in enclosed space. In measuring illumination level, the footcandle meter is held horizontally with its sensitive surface at least 30 centimeters from the body of the person holding the meter, The meter could be placed on a table and read from a distance to avoid obstruction of the light.
In conducting a general illumination check inside a room, the meter is held at least 80 centimeters above the floor. Reading is undertaken throughout the room and the results are recorded on the plan of the room.
a. All dimensions in meters
b. These spacing apply where desks and benches are next to wall, otherwise, one third the spacing between units is satisfactory.
c. The actual spacing of luminaries is usually less the maximum spacing to suit bay or room dimensions.
d. For mounting height of general diffusing and direct-indirect fixtures.
6-5 Uniformity of Light
The purpose of lighting calculations, by the foot-candle or lux, is to determine the average illumination in a room or lux, is to determine the average illumination in a room to a working level condition. This working level condition refers to the height of 75 centimeters above the floor being the approximate height of the table. The average illumination at the working level is directly related to the maximum spacing of the light to the mounting height ratio represented by the formula.
S/mh where: S = Spacing of light fixtures
mh = mounting height
Normally, the manufacture of light provides data with respect to spacing and mounting ratio. However, in the event that the manufacturer failed to provide these data, Table 6-8 was presented shoeing the spacing and mounting height ratio for particular lighting conditions.
Table 6-8 SPACING AND MOUNTING HEIGHT RATIO
A room with a ceiling height of 3 meters is to be lighted with direct concentrating fluorescent light. What is the maximum fixtures spacing?
1. For spacing and mounting ratio, refer to 6-8. The mounting height ratio od direct concentrating light is 0.40.
2. Substituting the given values, wherein mh is the ceiling height,
S = 0.40 x 3.00
Spacing: S = 1.20 meters maximum side to side of the fixtures.
A warehouse will install pendant dome incandescent lamps at a mounting ratio of 1.50 meters. The lamp will be mounted on a grid measuring 5.00 x 5.00 meters. What is the minimum mounting height of the lamps?
Mounting height is; mh = Spacing
mh = 5.00 m. = 3.30 meters
6-6 Classification of Lighting System
Lighting system is classified into four types, namely:
1. Direct lighting 2. Semi-direct lighting
3. Semi-indirect-lighting 4. Indirect Lighting
Direct Lighting. When the light on an illuminated area is focused downward coming directly from the lighting fixtures.
Semi-Direct Lighting. When the predominant light on the illuminated area is fed directly from the lighting units wherein the greater amount of light is obtained from the ceiling through the reflection.
Semi-Indirect Lighting. A lighting arrangement wherein 5% to 25% of the light is directed downward with more than half of the light focused upward and reflected from the ceiling.
Indirect Lighting. When the light is diffused and reflected from a wide ceiling area. This kind of lighting produces a soft and subdued effect due to low brightness and absence of sharp shadows.
CHART FOR ESTIMATING LIGHTING LOAD AND ILLUMINATION LEVEL CALCULATED FOR FAIRLY LARGE ROOM
6-7 Street Lighting
The Institute of Integrated Electrical Engineers instituted guidelines for adequate and acceptable illumination of the streets in order to promote safety. This concept was brought about by the continuously increasing speed of motor vehicles using the road.
The Philippine Electrical Code Committee prepared the guidelines for a standard practice on design of street lighting installation recommending the proper quantity and quality of light for traffic routes.
Definition of Terms
Lighting Installation – is defined as the whole of the equipment provided for lighting the roadway comprising the lamps luminaries, means of support and electrical installations including other auxiliaries.
Lighting System – refers to an array of luminaires having a characteristic of light distribution.
Luminaire – is a housing for one or more lamps comprising a body and any refractor, diffuser or enclosure associated with the lamps.
Road Width – is the distance between the edges of the road curbs measured at right angles to the length of the roadway.
Outreach – is the distance measured horizontally between the outer of the column or wall face or lamp post and the center of the luminaries.
Overhang – is the horizontal distance between the center of luminaires and the adjacent edge of the road.
Mounting Height – refers to the vertical distance between the center of the luminaire and the surface of the roadway.
Spacing – is the distance between the successive luminaries in an installation.
Maximum Light Utilization – In order to attain the maximum utilization of light from the fixtures, the luminaires should be mounted under the following specifications.
Luminance are properly selected and mounted on a location most feasible and effective with minimum cost. For a 230 volt system, a voltage drop of 5% is allowed although in extreme cases 15% voltage drop is sometimes tolerated.
For street illumination, the following formula is adopted.
E=The illumination in lux
Al=Average lumens with a typical value of:
20 500 lumens for 40 watts
11 500 lumens for 250 watts
5 400 lumens for 125 watts
The value of Al varies depending upon the type of lamp specified.
mf- is the maintenance factor which depends on the following:
a). Maintenance practice of the company
b). Operation of light sources at rate current and voltage
c). Regular replacement of depreciated lamp
d). Periodic cleaning of the luminaires either 0.8-0.9
w = Width of the roadway
d = Distance between luminaires
cu = Coefficient of utilization which is dependent on the type of fixture, mounting height, width of roadway and the lenght of mast arm or outreach.
The values given are based on the favorable reflectances for asphalt road, the recommended illumination should be increased by 50%. For concrete road, the recommended value could be decreased by 25%.
In decreasing street illumination, consider the modern lighting today that will be obsolete tomorrow when the minimum light levels are raised. The increasing motor vehicle speed and the incerasing congestiin in the street requires higher level of highway lighting. Therefore, future needs for light should be considered in the design.
Considering the data as presented on Figure 6-7 when the night pedestrian traffic is estimated oto be light and the night vehicular traffic is to be medium, determine the required lumens if theroad concrete is a pavement.
- Refering to Table 6- 9, E= 6.46 for light pedestrian medium traffic classifications. For concrete road, the reflectance will be higher but let us accept the value of 6.46 lumens.
- Determine the average pole distance.
E= 6.46 lumens per sq.m.
w= 7.00 meters
d= 50 meters
cu= 0.29 (type A fixture)
Under the Working Voltage, the mean lamp lumens of a 250 watts lamp is 11, 500 lumens, this is the nearest value to 8,662.83 average lumens. Therfore, a 250 watts lamp is acceptable.
Computing for the new actual illumination E
This is higher than the 6.46 recommended in table 6- 11. Therefore, the road is considered as adequeately lighted.